The V-belt puts much less tension on the motor bushings than the round rubber belt. The 15 clone didn't come with that round rubber band belt. It came with a V-Belt. Those rubber band belts came along later as a cheap and quick way to put a belt on the machines. They put a constant heavy load on the motor shaft bushings and can cause and accelerate wear on those parts.
The belt on the left is correct and preferred.
Joe